Aerodynamic drag due to viscous skin friction and/or pressure separation effects can severely reduce efficiency of vehicles during operation. Inability to obtain accurate wall shear stress measurements hinders better understanding of, and subsequently solving, this long-standing issue.
Although indirect methods for measuring wall shear stress, such as hotwires or particle image velocimetry (PIV) field visualization, are available, they are only primarily applicable in highly controlled testing environments, require extensive in situ calibration, and rely on inferred relations to produce a measurement. In some instances, hotwires may even disturb the turbulent flow to be measured by adding energy to the system.
Related art sensors are capable of directly measuring static or dynamic shear stress, and have achieved a noise floor of 14.9 μPa with 102 dB of dynamic range (U.S. Pat. No. 8,833,175 and Ref. [5]). These sensors, however, suffer from the drawback of single-axis measurements, limiting a real-time measurement of a 360-degree vector projection to 1 degree for any given measurement instance. In addition, accuracy in single-axis sensors is compromised by alignment in testing and multi-dimensional flow effects.